i am currently trying to solve the flow of fluid in a simple geometry with one velocity inlet and two pressure outlets. I want to implement a modified icoFoam solver that adjusts the pressure values at the outlets based on certain criteria (e.g., to achieve a certain predescribed flow split).
My main problem is that PISO seems to converge terribly slow. E.g., if I use a constant inlet velocity and constant pressure values at the outlets (i.e., steady-state), a relatively steady state is reached after around ten time steps, but both inlet pressure and outlet fluxes do not reach a constant value even after 100 time steps, although the changes are small in the end. Unfortunately, it seems that that many time steps are necessary whenever I change the outlet pressures.
So here are my questions:
- why does it take so long to converge? is there a way to accelerate this? I used timestep = 0.001; piso corrector steps did not have a significant effect.
Is it allowed to use PISO in this way?